1. Field of the Invention
This invention relates to extra-cardiac assist devices and more particularly to cardiac wraps that are selectively inflated and deflated to cyclically apply pressure to a treated heart.
2. Background Information
Ventricular enlargement is a clinically dangerous condition in which a ventricle of the patient""s heart increases in radius (e.g. dilates) until it is incapable of adequately pumping necessary blood through the patient""s body. Other equally life-threatening heart failure conditions may also result from severe heart attacks, with rapid deterioration of blood-pumping cap ability of the heart. A number of invasive procedures have been employed through the years in an effort to remedy ventricular dysfunction resulting from progressive acute conditions. Many of these procedures involve the use of cardiac assist devices that are implanted through the wall of the heart, and thereby come into direct contact with the bloodstream and internal vascular tissue. One potential disadvantage of such internally implanted devices is a chronic stimulus for blood clots or thrombosis and possible thromboembolism. This stimulus is present whenever artificial materials come in contact with blood, causing the accretion thereon of blood components. Thrombogenesis is typically controlled, or at least reduced through anticoagulation therapy. Such anticoagulants have several undesirable side effects, such as a higher propensity for bleeding.
Improved therapies and associated devices now exist that, under certain conditions, can replace internally implanted blood pumps. Ventricular dilation can be effectively treated using implants that are essentially free of contact with the heart""s internal, blood-contacting surfaces. U.S. Pat. No. 5, 800,528, entitled PASSIVE GIRDLE FOR HEART VENTRICLE FOR THERAPEUTIC AID TO PATIENTS HAVING VENTRICULAR DILATION by Lederman et al., expressly incorporated herein by reference, teaches one such treatment device. The device is applied over the affected ventricle or both affected ventricles by the clinician/surgeon, and held in place by small internal hooks (microhooks) or other fastening devices (sutures) that engage the outer wall of the heart. The girdle can be further constricted to better fit or compress the dilated ventricle using various mechanical systems such as pneumatic/hydraulic balloons or drawstrings. In this manner, the compression of the girdle passively (and continuously) counteracts excessive dilation of the ventricle.
U.S. Pat. No. 5,713,954, entitled EXTRA-CARDIAC VENTRICULAR ASSIST DEVICE by Rosenberg et al., also expressly incorporated herein by reference, teaches an active device that dynamically assists in the pumping of blood by the ventricles. Such an active device can treat both progressive and acutely failing heart conditions. This device is, like the girdle, placed around the affected ventricles and secured. A hydraulic or pneumatic (e.g. fluid) control system enables internal inflation elements (balloons) within the device to repetitively inflate and deflate, respectively squeezing and allowing expansion of the ventricle.
Both of the above-described exemplary devices employ an element termed a xe2x80x9cwrapxe2x80x9d that essentially wraps around the ventricular region of the heart. Active wraps inflate and deflate cyclically to assist the heart in the pumping of blood and may have a variety of shapes and sizes. They can be tailored to fit a particular affected heart, and accordingly, to operate to best treat the underlying condition of that heart.
The active extra-cardiac assist devices described above apply fluid-driven dynamic forces to expel blood from the ventricles of a heart. The forces are derived from either pneumatic or hydraulic actuation of the inflation elements so as to either apply the inflation pressure directly to the epicardium of the affected heart, or transduce the inflation pressure into a circumferential contraction about the heart. Devices operating under the former direct application principle are known as displacement devices, and generate maximally one unit of stroke volume in the heart for each corresponding unit of inflation volume. Devices operating under a contractile principle are not so limited, and in practice can achieve a relative gain in stroke volume in the heart of three to four times the inflation volume of the device. Because they require less volume, contractile devices also offer the opportunity for total device implantation. Displacement devices are powered by an extra-corporeal pressure/vacuum source.
In general, the self-contained balloons of the inflation elements are formed of a fatigue-resistant, biocompatible substance such as polyurethane. The inflation elements are located in pockets formed between a pair of fabric layers. A biocompatible fabric, such as woven polytetrafluoroethylene (PTFE) can be used. Pockets are defined by, for example, sewn seams. The seams typically consist of a strong, multifilament thread, formed from a biocompatible material such as Dupont Tyvek(copyright), that bridges or joins the above-mentioned biocompatible fabric. The fabric can be covered with a polymer or lightweight silicone coating to render it impermeable to fluid and tissue.
When a cardiac wrap is applied to an affected heart, it may enclose both the left and right ventricles to a large degree, applying a fixed or dynamic pressure to both simultaneously. However the afterload pressures (e.g. the pressure to eject blood from the ventricle) required in the right and left ventricles are not the same. The right ventricle exhibits a lower afterload pressure (approximately 20 mm/Hg) than that of the left ventricle (approximately 100 mm/Hg). Pressure applied to the whole ventricular surface (left and right) generally causes the right ventricle to empty first. In the case in which the left ventricle pumps less blood than the amount of blood entering the right ventricle, pulmonary edema results. Difference between right ventricular output and left ventricular output is balanced in the normal heart through the bronchial shunt, and by various changes in ventricular contractility based upon various neuro-hormonal feedback mechanisms. However, in unhealthy hearts, this balance may not be maintained, and needs to be provided for by a heart, assist device.
A number of current displacement and contractile cardiac wrap designs apply pressure to both ventricles simultaneously. The coverage area of these wraps for each of the left and right ventricles is roughly the same. However, the right ventricle pumps against significantly less afterload pressure to eject blood than the left ventricle due to the direct, low-resistance flow path from the right ventricle to the left ventricle via the pulmonary artery (compared to the aortic flow path serviced by the left ventricle. Therefore, the amount of applied contraction or displacement (for active wraps) to the right ventricle always occurs before the left ventricle is affected. In summary, existing wrap designs, that provide somewhat uniform pumping action across both the left and right ventricles, may over-pump the right ventricle in an effort to provide sufficient pumping action to the left ventricle, thereby defeating the balancing effect of the bronchial shunt. This can cause elevated right ventricular afterload, thereby increasing the threat of pulmonary edema.
It is therefore an object of this invention to provide a flow-balanced cardiac wrap in which right ventricular and left ventricular outputs are adjusted in order to prevent pulmonary edema or a worsening of ventricular function. Where the cardiac wrap is a permanent or semipermanent implantation, its inflation geometry should generally minimize the likelihood of bacterial colonization. The wrap should also provide improved fit and serviceability when applied to the affected heart.
This invention overcomes the disadvantages of the prior art by providing a flow-balanced cardiac wrap that assists the right and left ventricles of an affected heart to differing, and adjustable degrees. The wrap generally applies an assist to the left ventricle that is greater than that applied to the right, or that enables higher blood output from the left with respect to the right.
In one embodiment, the wrap comprises a material covering that is applied around the right and left ventricles of the heart, so that the left ventricle is assisted over a larger surface area than the right. The positioning of the right ventricular section of the wrap is chosen so that desired pumping characteristics for the right ventricle are achieved. For example, the positioning can be chosen to block a predetermined amount of blood flow within the right ventricle from reaching the heart valve and pulmonary artery. The wrap can include a pair of free ends with adjustable closures/connectors that are brought into engagement during implantation to complete the enclosure around the heart. The connectors can be arranged to enable selective placement of the right ventricular section. The wrap can be generally adapted to include adjustment elements, such as ties that allow the cone angle of the wrap with respect to the heart to be varied.
The wrap can include inflation elements positioned between material layers on the wrap to provide active assistance to the pumping action of the ventricles based upon the direction of fluid into the inflation elements. The placement, size and number of inflation elements can be varied relative to the left and right ventricular sections of the wrap. In addition, the pump and fluid interconnections can be adapted to provide different volumes of inflation fluid to each section, to deliver fluid at different flow rates, and/or to trigger inflation at different times. The inflation/deflation times can be set so that systole is held, and deflation of the overall wrap structure is delayed, with respect to the heart""s natural rhythm. The left ventricle can be displaced before the right and vice versa. Attentively, the right ventricle can be displaced every two cycles (e.g. every other cycle) with respect to displacement of the left ventricle.
According to another embodiment, the wrap can include dead spaces or semi-rigid members to alter the pressure-application characteristics of the wrap at certain locations. In yet another embodiment, the inflation elements can be moved about the circumference of a non-distensible wrap cover to apply selective displacement at different locations on the wrap.